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Harp seal
Here’s a familiar story. Humans systematically remove animals from natural populations, either in order to exploit what is seen as a resource, or to cull animals seen as a threat. When the cod stocks of the east coast of Canada started to decline in the 1980s there were calls to cull the harp seal and grey seal populations along the coast. It was said that the seals were taking cod and that culling the seals would increase the yield of the fishery. In other words, it wasn’t so much the decline in overall population that was stimulating the demand for a cull; it was the idea that if the seals took less, the fishery could take more.

Ecosystems don’t work in this simple manner. Seals will take other fish, too - maybe a lower predator that feeds on small cod. It is a question of establishing the food web that makes up an ecosystem and this may be very complicated, as the food web for the Benguela ecosystem off the coast of South Africa below demonstrates (Figure 3, p.637). Here, there are seals feeding on fish that are exploited commercially, but detailed modelling suggests that a cull of the seals would have a detrimental effect on yields from the exploited species.
Copyrighted imageCredit: 1999-2013 John Wiley & Sons, Inc.
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With a system as complicated as this you could not determine what might happen if you made a change, by doing an experiment. Only modelling offers a practical approach in this case.

The effects of change on some ecosystems have been studied experimentally, although modelling is still needed to predict long-term effects. These are systems in which top predators have been eliminated. A much studied example is in Yellowstone National Park. The top predator is the grey wolf but it was no longer present in the park after the 1920s. Wolves prey on elk all the year round, but once the pressure from predation was removed, the elk population enlarged and the principal cause of mortality was severe winters. Mortality became seasonal. Without wolves, the principal beneficiaries were scavengers, particularly those that could get to the carcasses even in the depths of winter.

In the last 60 years, winters in Yellowstone have been getting shorter. This change has an impact on the elk since fewer elk die and the period over which weather-linked deaths occur is a smaller proportion of the year. Scavengers have less time for feeding on elk carcasses and so probably moved to different prey at other times of the year. You can imagine that change spreading out across the whole ecosystem. In 1995, wolves were re-introduced and once again elk mortality was spread across the whole year.

As you can imagine, Yellowstone National Park is well-studied. In one study, 240 carcasses of elks killed by wolves were observed to see what which animals were feeding on them.
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Grey wolf
This was part of a study on the effects of climate change and one very interesting prediction that came out of the study was that the wolves would buffer the effect of warmer winters by maintaining a supply of carcasses for the scavengers all the year round. The decline in weather-related mortality of elks would be offset by wolf predation. So, regardless of whether the planet is warming or cooling, the presence of wolves will enable scavengers to adapt to the effect of global change, over a longer timescale.

Top predators are an integral part of an ecosystem, but recent work has highlighted their potential for ecosystem restoration and for providing ecosystems with resilience to changes in climate. Reintroduction of top predators is not easy or always desirable.

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